/*
 * Copyright (C) 2008 The Guava Authors
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package com.google.common.collect;

import static com.google.common.base.Preconditions.checkArgument;
import static com.google.common.base.Preconditions.checkNotNull;
import static com.google.common.collect.ObjectArrays.checkElementsNotNull;

import com.google.common.annotations.GwtCompatible;
import com.google.common.annotations.GwtIncompatible;
import com.google.errorprone.annotations.CanIgnoreReturnValue;
import com.google.errorprone.annotations.DoNotCall;
import com.google.errorprone.annotations.concurrent.LazyInit;

import java.io.InvalidObjectException;
import java.io.ObjectInputStream;
import java.io.Serializable;
import java.util.Arrays;
import java.util.Collection;
import java.util.Collections;
import java.util.Comparator;
import java.util.Iterator;
import java.util.NavigableSet;
import java.util.SortedSet;
import java.util.Spliterator;
import java.util.Spliterators;
import java.util.function.Consumer;
import java.util.stream.Collector;
import javax.annotation.CheckForNull;

import org.checkerframework.checker.nullness.qual.Nullable;

/**
 * A {@link NavigableSet} whose contents will never change, with many other important properties
 * detailed at {@link ImmutableCollection}.
 *
 * <p><b>Warning:</b> as with any sorted collection, you are strongly advised not to use a {@link
 * Comparator} or {@link Comparable} type whose comparison behavior is <i>inconsistent with
 * equals</i>. That is, {@code a.compareTo(b)} or {@code comparator.compare(a, b)} should equal zero
 * <i>if and only if</i> {@code a.equals(b)}. If this advice is not followed, the resulting
 * collection will not correctly obey its specification.
 *
 * <p>See the Guava User Guide article on <a href=
 * "https://github.com/google/guava/wiki/ImmutableCollectionsExplained"> immutable collections</a>.
 *
 * @author Jared Levy
 * @author Louis Wasserman
 * @since 2.0 (implements {@code NavigableSet} since 12.0)
 */
// TODO(benyu): benchmark and optimize all creation paths, which are a mess now
@GwtCompatible(serializable = true, emulated = true)
@SuppressWarnings("serial") // we're overriding default serialization
@ElementTypesAreNonnullByDefault
public abstract class ImmutableSortedSet<E> extends ImmutableSortedSetFauxverideShim<E>
        implements NavigableSet<E>, SortedIterable<E>
{
    static final int SPLITERATOR_CHARACTERISTICS =
            ImmutableSet.SPLITERATOR_CHARACTERISTICS | Spliterator.SORTED;

    /**
     * Returns a {@code Collector} that accumulates the input elements into a new {@code
     * ImmutableSortedSet}, ordered by the specified comparator.
     *
     * <p>If the elements contain duplicates (according to the comparator), only the first duplicate
     * in encounter order will appear in the result.
     *
     * @since 21.0
     */
    public static <E> Collector<E, ?, ImmutableSortedSet<E>> toImmutableSortedSet(
            Comparator<? super E> comparator)
    {
        return CollectCollectors.toImmutableSortedSet(comparator);
    }

    static <E> RegularImmutableSortedSet<E> emptySet(Comparator<? super E> comparator)
    {
        if (Ordering.natural().equals(comparator))
        {
            return (RegularImmutableSortedSet<E>) RegularImmutableSortedSet.NATURAL_EMPTY_SET;
        }
        else
        {
            return new RegularImmutableSortedSet<E>(ImmutableList.<E>of(), comparator);
        }
    }

    /**
     * Returns the empty immutable sorted set.
     *
     * <p><b>Performance note:</b> the instance returned is a singleton.
     */
    public static <E> ImmutableSortedSet<E> of()
    {
        return (ImmutableSortedSet<E>) RegularImmutableSortedSet.NATURAL_EMPTY_SET;
    }

    /**
     * Returns an immutable sorted set containing a single element.
     */
    public static <E extends Comparable<? super E>> ImmutableSortedSet<E> of(E element)
    {
        return new RegularImmutableSortedSet<E>(ImmutableList.of(element), Ordering.natural());
    }

    /**
     * Returns an immutable sorted set containing the given elements sorted by their natural ordering.
     * When multiple elements are equivalent according to {@link Comparable#compareTo}, only the first
     * one specified is included.
     *
     * @throws NullPointerException if any element is null
     */
    @SuppressWarnings("unchecked")
    public static <E extends Comparable<? super E>> ImmutableSortedSet<E> of(E e1, E e2)
    {
        return construct(Ordering.natural(), 2, e1, e2);
    }

    /**
     * Returns an immutable sorted set containing the given elements sorted by their natural ordering.
     * When multiple elements are equivalent according to {@link Comparable#compareTo}, only the first
     * one specified is included.
     *
     * @throws NullPointerException if any element is null
     */
    @SuppressWarnings("unchecked")
    public static <E extends Comparable<? super E>> ImmutableSortedSet<E> of(E e1, E e2, E e3)
    {
        return construct(Ordering.natural(), 3, e1, e2, e3);
    }

    /**
     * Returns an immutable sorted set containing the given elements sorted by their natural ordering.
     * When multiple elements are equivalent according to {@link Comparable#compareTo}, only the first
     * one specified is included.
     *
     * @throws NullPointerException if any element is null
     */
    @SuppressWarnings("unchecked")
    public static <E extends Comparable<? super E>> ImmutableSortedSet<E> of(E e1, E e2, E e3, E e4)
    {
        return construct(Ordering.natural(), 4, e1, e2, e3, e4);
    }

    /**
     * Returns an immutable sorted set containing the given elements sorted by their natural ordering.
     * When multiple elements are equivalent according to {@link Comparable#compareTo}, only the first
     * one specified is included.
     *
     * @throws NullPointerException if any element is null
     */
    @SuppressWarnings("unchecked")
    public static <E extends Comparable<? super E>> ImmutableSortedSet<E> of(
            E e1, E e2, E e3, E e4, E e5)
    {
        return construct(Ordering.natural(), 5, e1, e2, e3, e4, e5);
    }

    /**
     * Returns an immutable sorted set containing the given elements sorted by their natural ordering.
     * When multiple elements are equivalent according to {@link Comparable#compareTo}, only the first
     * one specified is included.
     *
     * @throws NullPointerException if any element is null
     * @since 3.0 (source-compatible since 2.0)
     */
    @SuppressWarnings("unchecked")
    public static <E extends Comparable<? super E>> ImmutableSortedSet<E> of(
            E e1, E e2, E e3, E e4, E e5, E e6, E... remaining)
    {
        Comparable[] contents = new Comparable[6 + remaining.length];
        contents[0] = e1;
        contents[1] = e2;
        contents[2] = e3;
        contents[3] = e4;
        contents[4] = e5;
        contents[5] = e6;
        System.arraycopy(remaining, 0, contents, 6, remaining.length);
        return construct(Ordering.natural(), contents.length, (E[]) contents);
    }

    // TODO(kevinb): Consider factory methods that reject duplicates

    /**
     * Returns an immutable sorted set containing the given elements sorted by their natural ordering.
     * When multiple elements are equivalent according to {@link Comparable#compareTo}, only the first
     * one specified is included.
     *
     * @throws NullPointerException if any of {@code elements} is null
     * @since 3.0
     */
    public static <E extends Comparable<? super E>> ImmutableSortedSet<E> copyOf(E[] elements)
    {
        return construct(Ordering.natural(), elements.length, elements.clone());
    }

    /**
     * Returns an immutable sorted set containing the given elements sorted by their natural ordering.
     * When multiple elements are equivalent according to {@code compareTo()}, only the first one
     * specified is included. To create a copy of a {@code SortedSet} that preserves the comparator,
     * call {@link #copyOfSorted} instead. This method iterates over {@code elements} at most once.
     *
     * <p>Note that if {@code s} is a {@code Set<String>}, then {@code ImmutableSortedSet.copyOf(s)}
     * returns an {@code ImmutableSortedSet<String>} containing each of the strings in {@code s},
     * while {@code ImmutableSortedSet.of(s)} returns an {@code ImmutableSortedSet<Set<String>>}
     * containing one element (the given set itself).
     *
     * <p>Despite the method name, this method attempts to avoid actually copying the data when it is
     * safe to do so. The exact circumstances under which a copy will or will not be performed are
     * undocumented and subject to change.
     *
     * <p>This method is not type-safe, as it may be called on elements that are not mutually
     * comparable.
     *
     * @throws ClassCastException   if the elements are not mutually comparable
     * @throws NullPointerException if any of {@code elements} is null
     */
    public static <E> ImmutableSortedSet<E> copyOf(Iterable<? extends E> elements)
    {
        // Hack around E not being a subtype of Comparable.
        // Unsafe, see ImmutableSortedSetFauxverideShim.
        @SuppressWarnings("unchecked")
        Ordering<E> naturalOrder = (Ordering<E>) Ordering.<Comparable>natural();
        return copyOf(naturalOrder, elements);
    }

    /**
     * Returns an immutable sorted set containing the given elements sorted by their natural ordering.
     * When multiple elements are equivalent according to {@code compareTo()}, only the first one
     * specified is included. To create a copy of a {@code SortedSet} that preserves the comparator,
     * call {@link #copyOfSorted} instead. This method iterates over {@code elements} at most once.
     *
     * <p>Note that if {@code s} is a {@code Set<String>}, then {@code ImmutableSortedSet.copyOf(s)}
     * returns an {@code ImmutableSortedSet<String>} containing each of the strings in {@code s},
     * while {@code ImmutableSortedSet.of(s)} returns an {@code ImmutableSortedSet<Set<String>>}
     * containing one element (the given set itself).
     *
     * <p><b>Note:</b> Despite what the method name suggests, if {@code elements} is an {@code
     * ImmutableSortedSet}, it may be returned instead of a copy.
     *
     * <p>This method is not type-safe, as it may be called on elements that are not mutually
     * comparable.
     *
     * <p>This method is safe to use even when {@code elements} is a synchronized or concurrent
     * collection that is currently being modified by another thread.
     *
     * @throws ClassCastException   if the elements are not mutually comparable
     * @throws NullPointerException if any of {@code elements} is null
     * @since 7.0 (source-compatible since 2.0)
     */
    public static <E> ImmutableSortedSet<E> copyOf(Collection<? extends E> elements)
    {
        // Hack around E not being a subtype of Comparable.
        // Unsafe, see ImmutableSortedSetFauxverideShim.
        @SuppressWarnings("unchecked")
        Ordering<E> naturalOrder = (Ordering<E>) Ordering.<Comparable>natural();
        return copyOf(naturalOrder, elements);
    }

    /**
     * Returns an immutable sorted set containing the given elements sorted by their natural ordering.
     * When multiple elements are equivalent according to {@code compareTo()}, only the first one
     * specified is included.
     *
     * <p>This method is not type-safe, as it may be called on elements that are not mutually
     * comparable.
     *
     * @throws ClassCastException   if the elements are not mutually comparable
     * @throws NullPointerException if any of {@code elements} is null
     */
    public static <E> ImmutableSortedSet<E> copyOf(Iterator<? extends E> elements)
    {
        // Hack around E not being a subtype of Comparable.
        // Unsafe, see ImmutableSortedSetFauxverideShim.
        @SuppressWarnings("unchecked")
        Ordering<E> naturalOrder = (Ordering<E>) Ordering.<Comparable>natural();
        return copyOf(naturalOrder, elements);
    }

    /**
     * Returns an immutable sorted set containing the given elements sorted by the given {@code
     * Comparator}. When multiple elements are equivalent according to {@code compareTo()}, only the
     * first one specified is included.
     *
     * @throws NullPointerException if {@code comparator} or any of {@code elements} is null
     */
    public static <E> ImmutableSortedSet<E> copyOf(
            Comparator<? super E> comparator, Iterator<? extends E> elements)
    {
        return new Builder<E>(comparator).addAll(elements).build();
    }

    /**
     * Returns an immutable sorted set containing the given elements sorted by the given {@code
     * Comparator}. When multiple elements are equivalent according to {@code compare()}, only the
     * first one specified is included. This method iterates over {@code elements} at most once.
     *
     * <p>Despite the method name, this method attempts to avoid actually copying the data when it is
     * safe to do so. The exact circumstances under which a copy will or will not be performed are
     * undocumented and subject to change.
     *
     * @throws NullPointerException if {@code comparator} or any of {@code elements} is null
     */
    public static <E> ImmutableSortedSet<E> copyOf(
            Comparator<? super E> comparator, Iterable<? extends E> elements)
    {
        checkNotNull(comparator);
        boolean hasSameComparator = SortedIterables.hasSameComparator(comparator, elements);

        if (hasSameComparator && (elements instanceof ImmutableSortedSet))
        {
            @SuppressWarnings("unchecked")
            ImmutableSortedSet<E> original = (ImmutableSortedSet<E>) elements;
            if (!original.isPartialView())
            {
                return original;
            }
        }
        @SuppressWarnings("unchecked") // elements only contains E's; it's safe.
        E[] array = (E[]) Iterables.toArray(elements);
        return construct(comparator, array.length, array);
    }

    /**
     * Returns an immutable sorted set containing the given elements sorted by the given {@code
     * Comparator}. When multiple elements are equivalent according to {@code compareTo()}, only the
     * first one specified is included.
     *
     * <p>Despite the method name, this method attempts to avoid actually copying the data when it is
     * safe to do so. The exact circumstances under which a copy will or will not be performed are
     * undocumented and subject to change.
     *
     * <p>This method is safe to use even when {@code elements} is a synchronized or concurrent
     * collection that is currently being modified by another thread.
     *
     * @throws NullPointerException if {@code comparator} or any of {@code elements} is null
     * @since 7.0 (source-compatible since 2.0)
     */
    public static <E> ImmutableSortedSet<E> copyOf(
            Comparator<? super E> comparator, Collection<? extends E> elements)
    {
        return copyOf(comparator, (Iterable<? extends E>) elements);
    }

    /**
     * Returns an immutable sorted set containing the elements of a sorted set, sorted by the same
     * {@code Comparator}. That behavior differs from {@link #copyOf(Iterable)}, which always uses the
     * natural ordering of the elements.
     *
     * <p>Despite the method name, this method attempts to avoid actually copying the data when it is
     * safe to do so. The exact circumstances under which a copy will or will not be performed are
     * undocumented and subject to change.
     *
     * <p>This method is safe to use even when {@code sortedSet} is a synchronized or concurrent
     * collection that is currently being modified by another thread.
     *
     * @throws NullPointerException if {@code sortedSet} or any of its elements is null
     */
    public static <E> ImmutableSortedSet<E> copyOfSorted(SortedSet<E> sortedSet)
    {
        Comparator<? super E> comparator = SortedIterables.comparator(sortedSet);
        ImmutableList<E> list = ImmutableList.copyOf(sortedSet);
        if (list.isEmpty())
        {
            return emptySet(comparator);
        }
        else
        {
            return new RegularImmutableSortedSet<E>(list, comparator);
        }
    }

    /**
     * Constructs an {@code ImmutableSortedSet} from the first {@code n} elements of {@code contents}.
     * If {@code k} is the size of the returned {@code ImmutableSortedSet}, then the sorted unique
     * elements are in the first {@code k} positions of {@code contents}, and {@code contents[i] ==
     * null} for {@code k <= i < n}.
     *
     * <p>If {@code k == contents.length}, then {@code contents} may no longer be safe for
     * modification.
     *
     * @throws NullPointerException if any of the first {@code n} elements of {@code contents} is null
     */
    static <E> ImmutableSortedSet<E> construct(
            Comparator<? super E> comparator, int n, E... contents)
    {
        if (n == 0)
        {
            return emptySet(comparator);
        }
        checkElementsNotNull(contents, n);
        Arrays.sort(contents, 0, n, comparator);
        int uniques = 1;
        for (int i = 1; i < n; i++)
        {
            E cur = contents[i];
            E prev = contents[uniques - 1];
            if (comparator.compare(cur, prev) != 0)
            {
                contents[uniques++] = cur;
            }
        }
        Arrays.fill(contents, uniques, n, null);
        return new RegularImmutableSortedSet<E>(
                ImmutableList.<E>asImmutableList(contents, uniques), comparator);
    }

    /**
     * Returns a builder that creates immutable sorted sets with an explicit comparator. If the
     * comparator has a more general type than the set being generated, such as creating a {@code
     * SortedSet<Integer>} with a {@code Comparator<Number>}, use the {@link Builder} constructor
     * instead.
     *
     * @throws NullPointerException if {@code comparator} is null
     */
    public static <E> Builder<E> orderedBy(Comparator<E> comparator)
    {
        return new Builder<E>(comparator);
    }

    /**
     * Returns a builder that creates immutable sorted sets whose elements are ordered by the reverse
     * of their natural ordering.
     */
    public static <E extends Comparable<?>> Builder<E> reverseOrder()
    {
        return new Builder<E>(Collections.reverseOrder());
    }

    /**
     * Returns a builder that creates immutable sorted sets whose elements are ordered by their
     * natural ordering. The sorted sets use {@link Ordering#natural()} as the comparator. This method
     * provides more type-safety than {@link #builder}, as it can be called only for classes that
     * implement {@link Comparable}.
     */
    public static <E extends Comparable<?>> Builder<E> naturalOrder()
    {
        return new Builder<E>(Ordering.natural());
    }

    /**
     * A builder for creating immutable sorted set instances, especially {@code public static final}
     * sets ("constant sets"), with a given comparator. Example:
     *
     * <pre>{@code
     * public static final ImmutableSortedSet<Number> LUCKY_NUMBERS =
     *     new ImmutableSortedSet.Builder<Number>(ODDS_FIRST_COMPARATOR)
     *         .addAll(SINGLE_DIGIT_PRIMES)
     *         .add(42)
     *         .build();
     * }</pre>
     *
     * <p>Builder instances can be reused; it is safe to call {@link #build} multiple times to build
     * multiple sets in series. Each set is a superset of the set created before it.
     *
     * @since 2.0
     */
    public static final class Builder<E> extends ImmutableSet.Builder<E>
    {
        private final Comparator<? super E> comparator;
        private E[] elements;
        private int n;

        /**
         * Creates a new builder. The returned builder is equivalent to the builder generated by {@link
         * ImmutableSortedSet#orderedBy}.
         */
        public Builder(Comparator<? super E> comparator)
        {
            super(true); // don't construct guts of hash-based set builder
            this.comparator = checkNotNull(comparator);
            this.elements = (E[]) new Object[ImmutableCollection.Builder.DEFAULT_INITIAL_CAPACITY];
            this.n = 0;
        }

        @Override
        void copy()
        {
            elements = Arrays.copyOf(elements, elements.length);
        }

        private void sortAndDedup()
        {
            if (n == 0)
            {
                return;
            }
            Arrays.sort(elements, 0, n, comparator);
            int unique = 1;
            for (int i = 1; i < n; i++)
            {
                int cmp = comparator.compare(elements[unique - 1], elements[i]);
                if (cmp < 0)
                {
                    elements[unique++] = elements[i];
                }
                else if (cmp > 0)
                {
                    throw new AssertionError(
                            "Comparator " + comparator + " compare method violates its contract");
                }
            }
            Arrays.fill(elements, unique, n, null);
            n = unique;
        }

        /**
         * Adds {@code element} to the {@code ImmutableSortedSet}. If the {@code ImmutableSortedSet}
         * already contains {@code element}, then {@code add} has no effect. (only the previously added
         * element is retained).
         *
         * @param element the element to add
         * @return this {@code Builder} object
         * @throws NullPointerException if {@code element} is null
         */
        @CanIgnoreReturnValue
        @Override
        public Builder<E> add(E element)
        {
            checkNotNull(element);
            copyIfNecessary();
            if (n == elements.length)
            {
                sortAndDedup();
                /*
                 * Sorting operations can only be allowed to occur once every O(n) operations to keep
                 * amortized O(n log n) performance.  Therefore, ensure there are at least O(n) *unused*
                 * spaces in the builder array.
                 */
                int newLength = ImmutableCollection.Builder.expandedCapacity(n, n + 1);
                if (newLength > elements.length)
                {
                    elements = Arrays.copyOf(elements, newLength);
                }
            }
            elements[n++] = element;
            return this;
        }

        /**
         * Adds each element of {@code elements} to the {@code ImmutableSortedSet}, ignoring duplicate
         * elements (only the first duplicate element is added).
         *
         * @param elements the elements to add
         * @return this {@code Builder} object
         * @throws NullPointerException if {@code elements} contains a null element
         */
        @CanIgnoreReturnValue
        @Override
        public Builder<E> add(E... elements)
        {
            checkElementsNotNull(elements);
            for (E e : elements)
            {
                add(e);
            }
            return this;
        }

        /**
         * Adds each element of {@code elements} to the {@code ImmutableSortedSet}, ignoring duplicate
         * elements (only the first duplicate element is added).
         *
         * @param elements the elements to add to the {@code ImmutableSortedSet}
         * @return this {@code Builder} object
         * @throws NullPointerException if {@code elements} contains a null element
         */
        @CanIgnoreReturnValue
        @Override
        public Builder<E> addAll(Iterable<? extends E> elements)
        {
            super.addAll(elements);
            return this;
        }

        /**
         * Adds each element of {@code elements} to the {@code ImmutableSortedSet}, ignoring duplicate
         * elements (only the first duplicate element is added).
         *
         * @param elements the elements to add to the {@code ImmutableSortedSet}
         * @return this {@code Builder} object
         * @throws NullPointerException if {@code elements} contains a null element
         */
        @CanIgnoreReturnValue
        @Override
        public Builder<E> addAll(Iterator<? extends E> elements)
        {
            super.addAll(elements);
            return this;
        }

        @CanIgnoreReturnValue
        @Override
        Builder<E> combine(ImmutableSet.Builder<E> builder)
        {
            copyIfNecessary();
            Builder<E> other = (Builder<E>) builder;
            for (int i = 0; i < other.n; i++)
            {
                add(other.elements[i]);
            }
            return this;
        }

        /**
         * Returns a newly-created {@code ImmutableSortedSet} based on the contents of the {@code
         * Builder} and its comparator.
         */
        @Override
        public ImmutableSortedSet<E> build()
        {
            sortAndDedup();
            if (n == 0)
            {
                return emptySet(comparator);
            }
            else
            {
                forceCopy = true;
                return new RegularImmutableSortedSet<E>(
                        ImmutableList.asImmutableList(elements, n), comparator);
            }
        }
    }

    int unsafeCompare(Object a, @CheckForNull Object b)
    {
        return unsafeCompare(comparator, a, b);
    }

    static int unsafeCompare(Comparator<?> comparator, Object a, @CheckForNull Object b)
    {
        // Pretend the comparator can compare anything. If it turns out it can't
        // compare a and b, we should get a CCE or NPE on the subsequent line. Only methods
        // that are spec'd to throw CCE and NPE should call this.
        @SuppressWarnings({"unchecked", "nullness"})
        Comparator<@Nullable Object> unsafeComparator = (Comparator<@Nullable Object>) comparator;
        return unsafeComparator.compare(a, b);
    }

    final transient Comparator<? super E> comparator;

    ImmutableSortedSet(Comparator<? super E> comparator)
    {
        this.comparator = comparator;
    }

    /**
     * Returns the comparator that orders the elements, which is {@link Ordering#natural()} when the
     * natural ordering of the elements is used. Note that its behavior is not consistent with {@link
     * SortedSet#comparator()}, which returns {@code null} to indicate natural ordering.
     */
    @Override
    public Comparator<? super E> comparator()
    {
        return comparator;
    }

    @Override // needed to unify the iterator() methods in Collection and SortedIterable
    public abstract UnmodifiableIterator<E> iterator();

    /**
     * {@inheritDoc}
     *
     * <p>This method returns a serializable {@code ImmutableSortedSet}.
     *
     * <p>The {@link SortedSet#headSet} documentation states that a subset of a subset throws an
     * {@link IllegalArgumentException} if passed a {@code toElement} greater than an earlier {@code
     * toElement}. However, this method doesn't throw an exception in that situation, but instead
     * keeps the original {@code toElement}.
     */
    @Override
    public ImmutableSortedSet<E> headSet(E toElement)
    {
        return headSet(toElement, false);
    }

    /**
     * @since 12.0
     */
    @Override
    public ImmutableSortedSet<E> headSet(E toElement, boolean inclusive)
    {
        return headSetImpl(checkNotNull(toElement), inclusive);
    }

    /**
     * {@inheritDoc}
     *
     * <p>This method returns a serializable {@code ImmutableSortedSet}.
     *
     * <p>The {@link SortedSet#subSet} documentation states that a subset of a subset throws an {@link
     * IllegalArgumentException} if passed a {@code fromElement} smaller than an earlier {@code
     * fromElement}. However, this method doesn't throw an exception in that situation, but instead
     * keeps the original {@code fromElement}. Similarly, this method keeps the original {@code
     * toElement}, instead of throwing an exception, if passed a {@code toElement} greater than an
     * earlier {@code toElement}.
     */
    @Override
    public ImmutableSortedSet<E> subSet(E fromElement, E toElement)
    {
        return subSet(fromElement, true, toElement, false);
    }

    /**
     * @since 12.0
     */
    @GwtIncompatible // NavigableSet
    @Override
    public ImmutableSortedSet<E> subSet(
            E fromElement, boolean fromInclusive, E toElement, boolean toInclusive)
    {
        checkNotNull(fromElement);
        checkNotNull(toElement);
        checkArgument(comparator.compare(fromElement, toElement) <= 0);
        return subSetImpl(fromElement, fromInclusive, toElement, toInclusive);
    }

    /**
     * {@inheritDoc}
     *
     * <p>This method returns a serializable {@code ImmutableSortedSet}.
     *
     * <p>The {@link SortedSet#tailSet} documentation states that a subset of a subset throws an
     * {@link IllegalArgumentException} if passed a {@code fromElement} smaller than an earlier {@code
     * fromElement}. However, this method doesn't throw an exception in that situation, but instead
     * keeps the original {@code fromElement}.
     */
    @Override
    public ImmutableSortedSet<E> tailSet(E fromElement)
    {
        return tailSet(fromElement, true);
    }

    /**
     * @since 12.0
     */
    @Override
    public ImmutableSortedSet<E> tailSet(E fromElement, boolean inclusive)
    {
        return tailSetImpl(checkNotNull(fromElement), inclusive);
    }

    /*
     * These methods perform most headSet, subSet, and tailSet logic, besides
     * parameter validation.
     */
    abstract ImmutableSortedSet<E> headSetImpl(E toElement, boolean inclusive);

    abstract ImmutableSortedSet<E> subSetImpl(
            E fromElement, boolean fromInclusive, E toElement, boolean toInclusive);

    abstract ImmutableSortedSet<E> tailSetImpl(E fromElement, boolean inclusive);

    /**
     * @since 12.0
     */
    @GwtIncompatible // NavigableSet
    @Override
    @CheckForNull
    public E lower(E e)
    {
        return Iterators.getNext(headSet(e, false).descendingIterator(), null);
    }

    /**
     * @since 12.0
     */
    @Override
    @CheckForNull
    public E floor(E e)
    {
        return Iterators.getNext(headSet(e, true).descendingIterator(), null);
    }

    /**
     * @since 12.0
     */
    @Override
    @CheckForNull
    public E ceiling(E e)
    {
        return Iterables.getFirst(tailSet(e, true), null);
    }

    /**
     * @since 12.0
     */
    @GwtIncompatible // NavigableSet
    @Override
    @CheckForNull
    public E higher(E e)
    {
        return Iterables.getFirst(tailSet(e, false), null);
    }

    @Override
    public E first()
    {
        return iterator().next();
    }

    @Override
    public E last()
    {
        return descendingIterator().next();
    }

    /**
     * Guaranteed to throw an exception and leave the set unmodified.
     *
     * @throws UnsupportedOperationException always
     * @since 12.0
     * @deprecated Unsupported operation.
     */
    @CanIgnoreReturnValue
    @Deprecated
    @GwtIncompatible // NavigableSet
    @Override
    @DoNotCall("Always throws UnsupportedOperationException")
    @CheckForNull
    public final E pollFirst()
    {
        throw new UnsupportedOperationException();
    }

    /**
     * Guaranteed to throw an exception and leave the set unmodified.
     *
     * @throws UnsupportedOperationException always
     * @since 12.0
     * @deprecated Unsupported operation.
     */
    @CanIgnoreReturnValue
    @Deprecated
    @GwtIncompatible // NavigableSet
    @Override
    @DoNotCall("Always throws UnsupportedOperationException")
    @CheckForNull
    public final E pollLast()
    {
        throw new UnsupportedOperationException();
    }

    @GwtIncompatible // NavigableSet
    @LazyInit
    @CheckForNull
    transient ImmutableSortedSet<E> descendingSet;

    /**
     * @since 12.0
     */
    @GwtIncompatible // NavigableSet
    @Override
    public ImmutableSortedSet<E> descendingSet()
    {
        // racy single-check idiom
        ImmutableSortedSet<E> result = descendingSet;
        if (result == null)
        {
            result = descendingSet = createDescendingSet();
            result.descendingSet = this;
        }
        return result;
    }

    // Most classes should implement this as new DescendingImmutableSortedSet<E>(this),
    // but we push down that implementation because ProGuard can't eliminate it even when it's always
    // overridden.
    @GwtIncompatible // NavigableSet
    abstract ImmutableSortedSet<E> createDescendingSet();

    @Override
    public Spliterator<E> spliterator()
    {
        return new Spliterators.AbstractSpliterator<E>(
                size(), SPLITERATOR_CHARACTERISTICS | Spliterator.SIZED)
        {
            final UnmodifiableIterator<E> iterator = iterator();

            @Override
            public boolean tryAdvance(Consumer<? super E> action)
            {
                if (iterator.hasNext())
                {
                    action.accept(iterator.next());
                    return true;
                }
                else
                {
                    return false;
                }
            }

            @Override
            public Comparator<? super E> getComparator()
            {
                return comparator;
            }
        };
    }

    /**
     * @since 12.0
     */
    @GwtIncompatible // NavigableSet
    @Override
    public abstract UnmodifiableIterator<E> descendingIterator();

    /**
     * Returns the position of an element within the set, or -1 if not present.
     */
    abstract int indexOf(@CheckForNull Object target);

    /*
     * This class is used to serialize all ImmutableSortedSet instances,
     * regardless of implementation type. It captures their "logical contents"
     * only. This is necessary to ensure that the existence of a particular
     * implementation type is an implementation detail.
     */
    private static class SerializedForm<E> implements Serializable
    {
        final Comparator<? super E> comparator;
        final Object[] elements;

        public SerializedForm(Comparator<? super E> comparator, Object[] elements)
        {
            this.comparator = comparator;
            this.elements = elements;
        }

        @SuppressWarnings("unchecked")
        Object readResolve()
        {
            return new Builder<E>(comparator).add((E[]) elements).build();
        }

        private static final long serialVersionUID = 0;
    }

    private void readObject(ObjectInputStream unused) throws InvalidObjectException
    {
        throw new InvalidObjectException("Use SerializedForm");
    }

    @Override
    Object writeReplace()
    {
        return new SerializedForm<E>(comparator, toArray());
    }
}
